1. Field of the Invention
The present invention relates to an apparatus for driving a brushless motor and method of controlling the motor and, more particularly, to an apparatus for driving a brushless motor and method of controlling the motor, which detects a position of a rotor based on a counter-electromotive voltage.
2. Description of the Related Art
In general, an output torque of a brushless motor is generated by an interaction between magnetic flux generated from magnetic poles of a rotor and a current flowing in a stator coil. Accordingly, to maximize the output torque, a current should be applied to the stator coil at a phase located near a position where the magnetic flux generated from the magnetic poles of the rotor is maximized.
However, since positions of the magnetic poles of the rotor change as the motor is rotated, a stator coil corresponding to a phase to which a current should be applied frequently changes. Accordingly, to apply the current to the phase of the stator coil corresponding to the positions of the magnetic poles of the rotor, it is necessary to perform phase commutation at an appropriate point of time.
Accordingly, phase commutation is performed by outputting a phase commutation signal and applying a current to a corresponding phase of the stator coil at a point of time when a predetermined delay time (generally, corresponding to an electrical angle of 30°) has elapsed after a counter-electromotive voltage induced from an unexcited stator coil has reached a predetermined zero-crossing point, as shown in FIG. 1.
In conventional methods, phase commutation is normally performed when a load current has a normal value. However, extinction of a current is not instantaneous due to an inductance component of the stator coil of the motor when the load current is greater than the normal value. In the latter case, a period To of FIG. 1 is lengthened, so that the counter-electromotive voltage is generated later than desired. Accordingly, a time required for the counter-electromotive voltage to reach the zero-crossing point is lengthened and phase commutation is performed later than desired. Therefore, a current is not applied, at an appropriate time, to the stator coil of a phase located near a position where magnetic flux generated from the magnetic poles of the rotor is maximized. Consequently, the output torque of the motor is decreased. In a worst case, the conventional methods are problematic in that the motor enters an out-of-step range where a rotation control of the motor is no longer performed and, therefore, the motor is stopped.